1. Field of the Invention
The present invention relates to a method and system for controlling an ultrasound scan sequence in Doppler signal detection.
2. Description of the Related Art
An ultrasound blood flow imaging apparatus employing an ultrasound Doppler method and a pulse reflection method is known to color-display, in real time, both blood flow and tomographic images of a subject acquired by an ultrasound probe. The principle of measurement of a blood flow velocity in the apparatus will be described below.
When an ultrasound beam is transmitted to blood flowing in a subject such as a living body, a center frequency fc of the ultrasound beam is Doppler-shifted by blood cells and is changed by a Doppler frequency fd. The frequency f of an ultrasound beam to be received is given as EQU f=fc+fd EQU for fd=2vcos.theta..multidot.fc/c
where v is the blood flow velocity, .theta. is the angle defined between an ultrasound beam transmission direction and a blood vessel direction, and c is the sound velocity. Therefore, the blood flow velocity v can be obtained upon detection of the Doppler frequency fd.
A two-dimensional image of the blood flow velocity v is displayed as follows. As shown in FIGS. 1 and 2, ultrasound beams are sequentially transmitted from an ultrasound probe 11 to a subject in scan directions A, B, C, D, . . . Z in response to pulse signals output from a transmitting circuit 17, and a sector scan is performed.
When an ultrasound beam is transmitted in the scan direction A, the center frequency of the ultrasound beam is Doppler-shifted by a blood flow in the subject. Echo signals reflected from the subject are received by the ultrasound probe 11 and are converted into electrical signals. These electrical signals are input to a receiving circuit 12.
A phase detecting circuit 13 detects a Doppler signal from the echo signal. This Doppler signal is detected at, e.g., 256 sample points set in each scan direction. The Doppler signal detected at each sample point is frequency-analyzed by a frequency analyzer 14 and is input to a display 16 through a digital scan converter (DSC) 15. Therefore, blood flow information in the scan direction A can be displayed on the display 16 in real time.
The same operations as described above are repeated in other scan directions B, C, D, . . . Z, and a blood flow velocity distribution image can be displayed based on the obtained Doppler signals.
A scan time tf required for obtaining one image is represented as follows: EQU tf=n.multidot.N/fr
where n is the scan count of each scan line, N is the number of scan lines, and fr is the ultrasound beam transmission frequency. For example, if n=8, N=16, and fr=4 kHz, then tf=8.times.16/4000=32 ms.
Display precision of the blood flow image is determined by the number of frames, the number of scan lines, and a field angle (in case of sector scan). It is, however, difficult to simultaneously optimize these parameters. For example, when the number of frames is increased to suppress flickering of a blood flow image to be displayed, the number of scan lines and the field angle are reduced. When the number of scan lines is increased to improve an image resolution, the field angle is reduced. When the field angle is increased to widen the scan range, the number of frames and the number of scan lines are reduced.
The above problems occur when the scan count of each scan line is maximized to detect blood flow information as precise as possible. When the scan count is reduced, the number of frames, the number of scan lines, and the field angle are increased. When the scan count is very small, however, a frequency analysis error is increased.
A method of simultaneously receiving a plurality of different echo signals is employed to optimally set the number of frames, the number of scan lines, and the field angle. According to this method, since echo signals from a subject are simultaneously received, a plurality of echo signals can be simultaneously received by one transmission of ultrasound beams. For example, when two echo signals are simultaneously received, two images can be acquired within the conventional image acquisition time. When the number of frames and the field angle are equal to those of a conventional system, the scan line interval can be halved. In addition, when the scan line interval is equal to that of the conventional system, the field angle can be doubled.
According to this method, a difference between data acquisition times of two scan lines upon one transmission of an ultrasound beam in a predetermined direction is zero. However, a difference Di in data acquisition times of two scan lines upon one transmission of an ultrasound beam in the next direction and those in the predetermined direction occurs. Therefore, the data acquisition time differences between scan lines are given as 0, Di, 0, Di, . . . and cannot be constant.
A data acquisition time difference between scan lines will be described with reference to an ultrasound scan sequence (scan count on one scan line is 4) for simultaneous reception in two directions, as shown in FIG. 3.
An ultrasound beam is transmitted from the ultrasound probe 11 at scan address 3 ( ), and ultrasound beams from the subject are simultaneously received at scan addresses 1 (.DELTA.) and 5 (.DELTA.), thereby obtaining reception data at scan addresses 2 (.largecircle.) and 4 (.largecircle.). This operation is repeated four times to obtain four reception data at each of scan addresses 2 and 4, and two display scan lines for the image display are formed.
An ultrasound beam is transmitted from the ultrasound probe 11 at scan address 7 ( ), and ultrasound beams from the subject are simultaneously received at scan addresses 5 (.DELTA.) and 9 (.DELTA.), thereby obtaining reception data at each of scan addresses 6 (.largecircle.) and 8 (.largecircle.). This operation is repeated four times to obtain four reception data at each of scan addresses 6 and 8, and two display scan lines for the image display are formed.
As can be apparent from the above description, although a data acquisition time difference between the display scan lines at scan addresses 2 and 4 is zero, that at scan addresses 4 and 6 corresponds to four transmission/reception periods. As described above, the data acquisition time differences between the display scan lines cannot be given to be constant. This indicates that scan is performed at irregular intervals, and a highly precise blood flow velocity distribution image cannot be obtained.
Under the above circumstances, a strong demand has arisen for an ultrasound diagnosis apparatus which can display a flow velocity distribution image by constant data acquisition times between scan lines using a method for a plural simultaneous reception.